A typical truck mounted crane or hoist, of the general class to which the invention relates, is shown in U.S. Pat. No. 3,082,889. Typically, such hoist will have a boom which may be of a fixed length or may be telescopic, mounted on top of a vertical post or column. The bottom end of the post or column will be mounted on or below the truck bed. Typically, some form of rotation mechanism is provided for rotating the column to swing the boom into the desired position. In the past, such rotation mechanism has usually incorporated some form of sprocket attached to the bottom end of the vertical column, and operated by means of a chain drive. The chain may be moved to and fro by means such as for example two hydraulic cylinders one at each end of the chain. Such a system is reasonably effective. However, it suffers from serious disadvantages.
It is desirable that hoists of this type may be made according to a standard design and then attached to various different makes of truck body, depending upon the customers requirements and choice.
The use of a chain and two hydraulic cylinders means that the two hydraulic must be attached on to the truck frame, or to the underside of the truck bed. Some form of attachment means must be provided which must be located precisely in relation to the position of the sprocket. Since there are wide variations in the design of truck bodies, it is thus necessary to have a variety of different attachment means. In addition, this factor also leads to problems in servicing, since the truck body may flex or move consequently changing the relationship between the chain and the sprocket. From all of these viewpoints it is therefore desirable that the entire hoist and rotation mechanism shall be built as far as possible as a single unit, which may then be attached to any suitable position on the truck body, without the requirement of different mounting means for different truck bodies.
In addition to these factors, it has also been found necessary to provide stabilizing legs which are extendable on opposite sides of the truck body so as to prevent the truck body from being tipped over by heavy loads carried on the boom. Typically stabilizing legs in the past have been attached to the truck bed as a separate attachment. They are usualy swingable in a vertical plane, and operated by hydraulic cylinders so that they may be swung upwardly out of the way or downwardly into contact with the ground. Again, the separate attachment of the stabilizing legs requires special engineering for a variety of different types of truck beds. It is therefore desirable that the stabilizing legs shall be incorporated in the design of the hoist itself so that the entire unit consisting of the hoist boom vertical column and stabilizing legs may be attached to the truck bed as a single integral unit. In addition, in this way it is possible for the stabilizing legs to actually stabilize the support of the vertical column of the hoist itself. In this way, there are no twisting stresses applied through the truck frame or truck bed between the location of the hoist on the one hand, and the location of the stabilizing legs on the other.